Valves of the heart including the aortic valve and mitral valve can become hardened from atherosclerotic plaque and calcium and no longer function normally. Alternately these valve can prolapse and allow blood to pass through the valve in a retrograde manner that is opposite to the normal direction of flow through the valve. Such regurgitant flow can require repair or replacement of the valve. Surgical repair or replacement of such valve is often the gold standard at present for those patients able to withstand the rigors of surgery. An alternate and less invasive approach would be desirable via access to the valve from the femoral vasculature, vasculature of the arms, the apex of the heart, aortic access, or via other less invasive sites.
Transcatheter aortic valve replacement (TAVR) has evolved to become an accepted less invasive procedure for replacing diseased or incompetent aortic valves in high risk patients. Such less invasive surgical procedures are not as well developed for replacing abnormally functioning mitral valves.
Often the regurgitant mitral valve is a result of excessive expansion of the left ventricle (LV) leading to abnormal tension and angulation imposed on the mitral valve leaflet. The mitral valve leaflet is often unable to coapt properly with its neighboring leaflet and will therein allow retrograde blood flow to occur through the valve. The mitral valve annulus is more elastic, in part, than the aortic annulus and can expand in diameter reducing the ability of the mitral valve leaflets to coapt properly; one should not expand a stent into the mitral annulus to push it further outwards as is done with TAVR procedures onto the aortic valve annulus.
The mitral anatomy also provides that the anterior mitral leaflet not only helps close the mitral annulus during systole, but also provides one surface of the left ventricular outflow track (LVOT) during systolic pumping of blood out of the LV. It is therefore not acceptable to expand a stent indiscriminately outwards as is done in TAVR due to the potential for blockage of the LVOT by the anterior mitral valve leaflet.
The use of barbs or other fixation members to hold the TMVR device securely to the native mitral apparatus can create a set of potential clinical issues that are problematic to the patient. Expansion of barbs prior to full apposition of the TMVR stent against the mitral annulus, for example, can obviate the ability of the barbs to position themselves and the stent-valve frame uniformly around the perimeter of the mitral annulus. Furthermore, activation of barbs via a standard dilation balloon can block blood flow through the mitral annulus during balloon inflation causing the patient to temporarily go without oxygen supply to the brain with its ensuing consequences. Additionally, inflation of a standard balloon can cause the positioning of the stent-valve to become instantaneously displaced and hence inappropriately located across the mitral annulus due to blood pressure and blood flow generated by the LV.
The delivery profile of TMVR devices is generally greater than those for TAVR due to the larger diameter of the mitral annulus in comparison to the aortic annulus. This profile limitation has forced many of the TMVR devices to be delivered via the apex of the heart rather than through a more favorable transvascular and transseptal delivery approach. The apical approach is not well suited to patients that are older in age or are of higher risk. What is needed is a TMVR device that is of a lower profile such that it can be delivered via a transvascular and transseptal approach. The device should be easily positioned across the mitral annulus and secured to the native mitral apparatus without chance for device migration. The TMVR device should eliminate regions for blood stagnation that can lead to thromboemboli that could potentially result in stroke and should not restrict blood flow out of the LVOT.